Bifocal optical assembly for a head-mounted display

ABSTRACT

A head-mounted display (HMD) presented herein comprises an electronic display and an optical assembly. The electronic display is configured to emit image light. The optical assembly is configured to direct the image light to an eye-box of the HMD corresponding to a location of a user&#39;s eye. The optical assembly includes a multifocal optical element, e.g., a bifocal optical element. A first portion of the multifocal optical element has a first optical power that is associated with a first image plane. The second portion of the multifocal optical element has a second optical power different than the first optical power, the second portion associated with a second image plane.

BACKGROUND

The present disclosure generally relates to displaying content to a userwearing a head-mounted display (HMD) as part of an artificial realitysystem, and specifically relates to a bifocal optical assembly for theHMD.

Vergence-accommodation conflict is a phenomenon that occurs to users ofvirtual headsets such as HMDs. Typically, eyes converge (rotate towardone another) to focus on closer objects and diverge (rotate away fromone another) to focus on objects that are further away. The vergencetherefore represents the simultaneous movement of both eyes in oppositedirections to obtain or maintain single binocular vision. Accommodationis coupled with vergence, and is the process where the lenses of theeyes focus on a close or far away object. During accommodation of aneye, a crystalline lens of the eye changes optical power to maintain aclear image or focus on an object as the object's distance varies. InHMD systems, vergence and accommodation processes are decoupled. Infixed-focused HMD systems, the user's eyes verge to a virtual object,but the accommodation stimulus is incorrect for near objects. The eyesmay accommodate to the fixed focus distance of a display in the HMD,conflicting with the verged distance to the virtual object. More oftenthe eyes will accommodate for a near object, which causes image blursince the virtual object distance is fixed. The decoupling of vergenceand accommodation processes can cause the user to feel uncomfortable,disoriented, or nauseous. Furthermore, different users wearing the sameHMD have different accommodation abilities, e.g., in accordance with anage of a user. In general, older people have less ability to accommodatethan younger people, i.e., an accommodative range of older people issmaller than that of younger people. Therefore, it is desirable todesign an optical assembly for integration into a HMD that canefficiently drive the accommodation for different users, which wouldalso mitigate the vergence-accommodation conflict.

SUMMARY

Embodiments of the present disclosure support a head-mounted display(HMD) comprising an electronic display and an optical assembly. Theelectronic display is configured to emit image light. The opticalassembly is configured to direct the image light to an eye-box of theHMD corresponding to a location of a user's eye. The optical assemblyincludes a multifocal optical element, e.g., a bifocal optical element.A first portion of the multifocal optical element has a first opticalpower that is associated with a first image plane, and a second portionof the multifocal optical element has a second optical power differentthan the first optical power and the second optical power is associatedwith a second image plane. The multifocal optical element providescontent to the user in at least two image planes in order to allow fordifferent accommodative ranges of different users. In some embodiments,the multifocal optical element may also mitigate vergence-accommodationconflict. Additionally, in some embodiments, the multifocal opticalelement optically corrects the image light before directing it to theeye-box.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows the relationship between vergence and eye focal length(accommodation) in the real world.

FIG. 1B shows the conflict between vergence and eye focal length(accommodation) in a three-dimensional display screen.

FIG. 2A is a diagram of a head-mounted display (HMD), in accordance withone or more embodiments.

FIG. 2B is a cross section of a front rigid body of the HMD in FIG. 2A,in accordance with one or more embodiments.

FIG. 3A is an example cross section of an optical assembly with abifocal optical element coupled to an electronic display, which may bepart of the HMD in FIG. 2A, in accordance with one or more embodiments.

FIG. 3B shows examples of a bifocal optical element, in accordance withone or more embodiments.

FIG. 3C shows an example cross-section of a bifocal optical element andpropagation of image light through the bifocal optical element for anear-focused eye, in accordance with one or more embodiments.

FIG. 3D shows an example cross-section of a bifocal optical element andpropagation of image light through the bifocal optical element for afar-focused eye, in accordance with one or more embodiments.

FIG. 4 is an example design of a multi-focal optical element implementedas a progressive lens, in accordance with one or more embodiments.

FIG. 5 is an example cross section of an optical assembly with a bifocaloptical element implemented as a bifocal insert, in accordance with oneor more embodiments.

FIG. 6A is an example cross section of an optical assembly in opticalseries with a tilted display for achieving lower field myopia, inaccordance with one or more embodiments.

FIG. 6B is an example cross section of an optical assembly in opticalseries with a curved (bent) display for achieving lower field myopia, inaccordance with one or more embodiments.

FIG. 7A is an example cross section of an optical assembly in opticalseries with a display having a wedge for achieving lower field myopia,in accordance with one or more embodiments.

FIG. 7B is another example cross section of an optical assembly inoptical series with a display having a wedge for achieving lower fieldmyopia, in accordance with one or more embodiments.

FIG. 8 is an example cross section of an optical assembly in opticalseries with a display having a transparent optical element for achievinglower field myopia, in accordance with one or more embodiments.

FIG. 9 is an example cross section of an optical assembly in opticalseries with a display having a fiber optic faceplate for achieving lowerfield myopia, in accordance with one or more embodiments.

FIG. 10 is a block diagram of a HMD system in which a console operates,in accordance with one or more embodiments.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

Embodiments of the present disclosure may include or be implemented inconjunction with an artificial reality system. Artificial reality is aform of reality that has been adjusted in some manner beforepresentation to a user, which may include, e.g., a virtual reality (VR),an augmented reality (AR), a mixed reality (MR), a hybrid reality, orsome combination and/or derivatives thereof. Artificial reality contentmay include completely generated content or generated content combinedwith captured (e.g., real-world) content. The artificial reality contentmay include video, audio, haptic feedback, or some combination thereof,and any of which may be presented in a single channel or in multiplechannels (such as stereo video that produces a three-dimensional effectto the viewer). Additionally, in some embodiments, artificial realitymay also be associated with applications, products, accessories,services, or some combination thereof, that are used to, e.g., createcontent in an artificial reality and/or are otherwise used in (e.g.,perform activities in) an artificial reality. The artificial realitysystem that provides the artificial reality content may be implementedon various platforms, including a HMD connected to a host computersystem, a standalone HMD, a mobile device or computing system, or anyother hardware platform capable of providing artificial reality contentto one or more viewers.

A HMD displays content to a user. The HMD may be part of an artificialreality system. The HMD includes an optical assembly and an electronicdisplay. In some embodiments, the optical assembly of the HMD includes abifocal optical element. The bifocal optical element has a specificoptical power except for a portion of the bifocal optical element thatis formed to have less optical power. Thus, the portion of the bifocaloptical element is implemented as a power reducer. In some embodiments,the power reducer is positioned below an optical axis of the HMD. Forexample, the user of the HMD would gaze down (i.e., below the opticalaxis) in order to look through the power reducer. The content presentedthrough the power reducer allow users of different accommodative rangesto view content in at least a first image plane and a second imageplane, i.e., an image plane for content not viewed through the powerreducer and an image plane for other content viewed through the powerreducer. The power reducer does this by setting an accommodative rangebetween the first and second image plane such that a broader range ofusers (e.g., children and adults) are able to focus on either imageplane. In this way, the bifocal element generates two separate imageplanes that are at different image distances. Users having differentranges of accommodation (e.g., an adult and a child) are able to focuson both of the image planes, thereby expanding a size of a user base forthe HMD system. Moreover, as the bifocal element is a passive device, itis relatively simple and has a small form factor, both of which areadvantages in implanting the bifocal element in an HMD. Additionally,the bifocal element may also mitigate vergence-accommodation conflict.

Vergence-accommodation conflict is a problem in many virtual realitysystems. Vergence is the simultaneous movement or rotation of both eyesin opposite directions to obtain or maintain single binocular vision andis linked to accommodation of the eye. Under normal conditions, whenhuman eyes change fixation from one object to another object at adistance, the eyes automatically change focus (by changing the shape ofthe crystalline lens) to provide accommodation at the new distance orvergence depth of the new object. Furthermore, in general, older peoplehave less ability to accommodate than younger people, i.e., older peoplein general have a smaller accommodative range than younger people.Therefore, older users wearing a HMD typically would not need to look atcontent presented on a display through the power reducer. However,younger people having a larger accommodative range should view contentthat appears further through the power reducer in order to betteraccommodate when they continue viewing content that appears closer.

In some embodiments, to mitigate vergence-accommodation conflict, theelectronic display of the HMD is configured to present content in twoimage planes. The electronic display can be implemented to have a nearand far focal portion to mitigate vergence-accommodation conflict inboth users with a large range of accommodation as well as users with amore limited range of accommodation. The near focal portion is generallypositioned in a lower portion of a field-of-view of the HMD. In oneembodiment, the electronic display provides two image planes by tilting(or bending) the electronic display with respect to the opticalassembly. In other embodiment, the electronic display provides two imageplanes by bending the electronic display with respect to the opticalassembly. In yet other embodiments, a fiber taper (also referred to as afiber optic faceplate) or a wedge (both of which may be shaped) that iscoupled to the electronic display may be used to provide a localizedshift in virtual image distance. In one or more embodiments, the opticalassembly may include a lens that includes a region having a reducedoptical power with respect to the rest of the lens. The region ofreduced optical power may correspond to the near focal portion.

FIG. 1A shows the relationship between vergence and eye focal length(accommodation) in the real world. In the example 100 of FIG. 1A, theuser is looking at a real scene 105 (e.g., that includes one or morereal objects) located far from the user, i.e., the scene 105 located farfrom the user is in focus. In the same time, user's eyes 110 are vergedon the far real scene 105 and gaze lines from the user's eyes 110intersect at the real scene 105. Thus, the vergence distance (d_(v))equals the accommodation (focal) distance (d_(f)) in the example 100. Inthe example 120 of FIG. 1A, when the real scene 105 (e.g., the one ormore real objects) is moved closer to the user, as indicated by thearrow in FIG. 1A, each eye 110 rotates inward (i.e., converge) to stayverged on the real scene 105. As the real scene 105 gets closer in theexample 120, the eyes 110 have to “accommodate” to the closer distanceby changing the shape of the crystalline lens to reduce the focal lengthor increase the optical power. Thus, under normal conditions in the realworld (and for young people), the vergence distance (d_(v)) still equalsthe accommodation distance (d_(f)) when looking into near objects, i.e.,the vergence and accommodation are coupled in the real world.

FIG. 1B shows an example conflict between vergence and accommodationthat can occur with stereoscopic three-dimensional displays, e.g.,fixed-focus HMDs. In the example 130 of FIG. 1B, a user is looking at avirtual scene 135 (e.g., composed of one or more virtual objects)displayed on a 3D electronic display 140. An eyepiece 145 may bepositioned in front of each eye 110. The user's eyes 110 are verged onand gaze lines from the user's eyes 110 intersect at the virtual scene135, which is at a greater distance from the user's eyes 110 than the 3Delectronic display 140. In the example 150 of FIG. 1B, the virtual scene135 is rendered on the 3D electronic display 140 to appear closer to theuser, each eye 110 again rotates inward (e.g., as shown by arrows inFIG. 1B) to stay verged on the virtual scene 135. The eyes 110accommodate as usual when verging to the near virtual scene 135, but theaccommodation stimulus is incorrect (e.g., for young people) and thefocus distance of the image is not sufficiently reduced. Thus, the focusis still distant and the virtual scene 135 presented on the 3Delectronic display 140 appears blurry. Thus, instead of increasing theoptical power to accommodate for the closer vergence depth, the eyes 110maintain accommodation at a distance associated with the 3D electronicdisplay 140. This discrepancy between vergence depth and focal lengthcaused by decoupling of vergence and accommodation in stereoscopicthree-dimensional displays is referred to as “vergence-accommodationconflict.” A user experiencing only vergence or accommodation and notboth will eventually experience some degree of fatigue and nausea, whichis undesirable for virtual reality system creators. Embodiments of thepresent disclosure relate to an optical assembly configured to correctfor optical power during accommodation in the fixed-focus HMDs andmitigate vergence-accommodation conflict for users of different age. Theoptical assembly correcting for optical power during accommodation maybe part of the eyepiece 145.

FIG. 2A is a diagram of a HMD 200, in accordance with one or moreembodiments. The HMD 200 may be part of an artificial reality system. Inembodiments that describe an AR system and/or a MR system, portions of afront side 202 of the HMD 200 are at least partially transparent in thevisible band (˜380 nm to 750 nm), and portions of the HMD 200 that arebetween the front side 202 of the HMD 200 and an eye of the user are atleast partially transparent (e.g., a partially transparent electronicdisplay). The HMD 200 includes a front rigid body 205, a band 210, and areference point 215. The HMD 200 may also include a depth cameraassembly (DCA) configured to determine depth information of a local areasurrounding some or all of the HMD 200. The HMD 200 may also include animaging aperture 220 and an illumination aperture 225, and anillumination source of the DCA emits light (e.g., a structured lightpattern) through the illumination aperture 225. An imaging device of theDCA captures light from the illumination source that is reflected fromthe local area through the imaging aperture 220.

The front rigid body 205 includes one or more electronic displayelements (not shown in FIG. 2A), one or more integrated eye trackingsystems (not shown in FIG. 2), an Inertial Measurement Unit (IMU) 230,one or more position sensors 235, and the reference point 215. In theembodiment shown by FIG. 2A, the position sensors 235 are located withinthe IMU 230, and neither the IMU 230 nor the position sensors 235 arevisible to a user of the HMD 200. The IMU 230 is an electronic devicethat generates IMU data based on measurement signals received from oneor more of the position sensors 235. A position sensor 235 generates oneor more measurement signals in response to motion of the HMD 200.Examples of position sensors 235 include: one or more accelerometers,one or more gyroscopes, one or more magnetometers, another suitable typeof sensor that detects motion, a type of sensor used for errorcorrection of the IMU 230, or some combination thereof. The positionsensors 235 may be located external to the IMU 230, internal to the IMU230, or some combination thereof.

FIG. 2B is a cross section 240 of the front rigid body 205 of the HMD200 shown in FIG. 2A. As shown in FIG. 2B, the front rigid body 205includes an electronic display 245 and an optical assembly 250 thattogether provide image light to an eye-box 255. The eye-box 255 is aregion in space that is occupied by a user's eye 260. For purposes ofillustration, FIG. 2B shows a cross section 240 associated with a singleeye 260, but another optical assembly 250, separate from the opticalassembly 250, provides altered image light to another eye of the user.

The electronic display 245 generates image light. In some embodiments,the electronic display 245 includes an optical element that adjusts thefocus of the generated image light. The electronic display 245 displaysimages to the user in accordance with data received from a console (notshown in FIG. 2B). In various embodiments, the electronic display 245may comprise a single electronic display or multiple electronic displays(e.g., a display for each eye of a user). Examples of the electronicdisplay 245 include: a liquid crystal display (LCD), an organic lightemitting diode (OLED) display, an inorganic light emitting diode (ILED)display, an active-matrix organic light-emitting diode (AMOLED) display,a transparent organic light emitting diode (TOLED) display, some otherdisplay, a projector, or some combination thereof. The electronicdisplay 245 may also include an aperture, a Fresnel lens, a convex lens,a concave lens, a diffractive element, a waveguide, a filter, apolarizer, a diffuser, a fiber taper, a reflective surface, a polarizingreflective surface, or any other suitable optical element that affectsthe image light emitted from the electronic display. In someembodiments, one or more of the display block optical elements may haveone or more coatings, such as anti-reflective coatings. In someembodiments, the electronic display 245 is configured to present contentin two image planes in order to mitigate vergence-accommodation conflictin both users with a large range of accommodation as well as users witha more limited range of accommodation. More details about the electronicdisplay 245 configured to provide variable accommodative ranges aredisclosed in conjunction with FIGS. 6A-6B, FIGS. 7A-7B, FIG. 8, and FIG.9.

The optical assembly 250 magnifies received light from the electronicdisplay 245, corrects optical aberrations associated with the imagelight, and the corrected image light is presented to a user of the HMD200. At least one optical element of the optical assembly 250 may be anaperture, a Fresnel lens, a refractive lens, a reflective surface, adiffractive element, a waveguide, a filter, or any other suitableoptical element that affects the image light emitted from the electronicdisplay 245. Moreover, the optical assembly 250 may include combinationsof different optical elements. In some embodiments, one or more of theoptical elements in the optical assembly 250 may have one or morecoatings, such as anti-reflective coatings, dichroic coatings, etc.Magnification of the image light by the optical assembly 250 allowselements of the electronic display 245 to be physically smaller, weighless, and consume less power than larger displays. Additionally,magnification may increase a field-of-view of the displayed media. Forexample, the field-of-view of the displayed media is such that thedisplayed media is presented using almost all (e.g., 110 degreesdiagonal), and in some cases all, of the field-of-view. In someembodiments, the optical assembly 250 is designed so its effective focallength is larger than the spacing to the electronic display 245, whichmagnifies the image light projected by the electronic display 245.Additionally, in some embodiments, the amount of magnification may beadjusted by adding or removing optical elements. In some embodiments,the optical assembly 250 includes a multifocal optical element (e.g., abifocal optical element) for providing variable accommodative ranges inorder to mitigate vergence-accommodation conflict. More details aboutthe optical assembly 250 with the multifocal optical element aredisclosed in conjunction with FIGS. 3A-3D, FIG. 4, and FIG. 5.

In some embodiments, the front rigid body 205 further includes a DCA 265for determining depth information of one or more objects in a local area270 surrounding some or all of the HMD 200. The DCA 265 includes a lightgenerator 275, an imaging device 280, and a controller 285 that may becoupled to both the light generator 275 and the imaging device 280. Thelight generator 275 emits light through the illumination aperture 225.The light generator 275 illuminates the local area 270 with illuminationlight 290, e.g., in accordance with emission instructions generated bythe controller 285. The controller 285 is configured to control, basedon the emission instructions, operation of certain components of thelight generator 275, e.g., to adjust an intensity and a pattern of theillumination light 290 illuminating the local area 270.

The light generator 275 may include a plurality of emitters that eachemits light having certain characteristics (e.g., wavelength,polarization, coherence, temporal behavior, etc.). The characteristicsmay be the same or different between emitters, and the emitters can beoperated simultaneously or individually. In one embodiment, theplurality of emitters could be, e.g., laser diodes (e.g., edgeemitters), inorganic or organic LEDs, a vertical-cavity surface-emittinglaser (VCSEL), or some other source. In some embodiments, a singleemitter or a plurality of emitters in the structured light generator 275can emit one or more light beams.

The imaging device 280 includes one or more cameras configured tocapture, through the imaging aperture 220, at least a portion of theillumination light 290 reflected from the local area 270. The imagingdevice 280 captures one or more images of one or more objects in thelocal area 270 illuminated with the illumination light 290. Thecontroller 285 coupled to the imaging device 280 is also configured todetermine depth information for the one or more objects based on thecaptured portion of the reflected illumination light. In someembodiments, the controller 285 provides the determined depthinformation to a console (not shown in FIG. 2B) and/or an appropriatemodule of the HMD 200 (e.g., a varifocal module, not shown in FIG. 2B).The console and/or the HMD 200 may utilize the depth information to,e.g., generate content for presentation on the electronic display 245.

In some embodiments, the front rigid body 205 further comprises an eyetracking system (not shown in FIG. 2B) that determines eye trackinginformation for the user's eye 260. The determined eye trackinginformation may comprise information about an orientation of the user'seye 260 in in eye-box 255, i.e., information about an angle of aneye-gaze. In one embodiment, the user's eye 260 is illuminated withstructured light. Then, the eye tracking system can use locations of thereflected structured light in a captured image to determine eye positionand eye-gaze. In another embodiment, the eye tracking system determineseye position and eye-gaze based on magnitudes of image light capturedover a plurality of time instants.

In some embodiments, the front rigid body 205 further comprises avarifocal module (not shown in FIG. 2B). The varifocal module may adjustfocus of one or more images displayed on the electronic display 245,based on the eye tracking information. In one embodiment, the varifocalmodule adjusts focus of the displayed images and mitigatesvergence-accommodation conflict by adjusting a focal distance of theoptical assembly 250 based on the determined eye tracking information.In another embodiment, the varifocal module adjusts focus of thedisplayed images by performing foveated rendering of the one or moreimages based on the determined eye tracking information. In yet anotherembodiment, the varifocal module utilizes the depth information from thecontroller 285 to generate content for presentation on the electronicdisplay 245.

FIG. 3A is an example cross section 300 of an optical assembly 305coupled to an electronic display 310, which may be part of the HMD 200in FIG. 2A, in accordance with one or more embodiments. The opticalassembly 305 may be an embodiment of the optical assembly 250 of FIG.2B. The optical assembly 305 may provide optical correction to imagelight emitted from different portions of the electronic display 310 andmay direct the optically corrected image light to an eye-box 315 of aHMD corresponding to a location of a user's eye 320. In someembodiments, the optical assembly 305 includes a bifocal optical element325 having a first optical power in a first portion 330 and a secondoptical power in a second portion 335. In one or more embodiments, thesecond portion 335 of the bifocal optical element 325 is implemented tohave less optical power than the first portion 330. The bifocal opticalelement 325 receives and optically corrects image light emitted fromdifferent portions of the electronic display 310. The bifocal opticalelement 325 further directs the optically corrected image light to theeye 320 in two image planes in order to mitigate vergence-accommodationconflict in users having different accommodation ranges, as discussed inmore detail below.

The electronic display 310 emits content (image light) that is receivedand optically corrected by the bifocal optical element 325. Theelectronic display 310 may be an embodiment of the electronic display245 of FIG. 2B. The electronic display 310 may emit image light 340 froma portion of the electronic display 310 located above an optical axis342, wherein the image light 340 may correspond to a far virtual imagedisplayed on this portion of the electronic display 310. The firstportion 330 of the bifocal optical element 325 may direct the imagelight 340 to at least one surface of the eye 320 as image light 345.Thus, the image light 345 provides the far virtual image to the eye 320in a first image plane, wherein the eye 320 looking through the firstportion 330 of the bifocal optical element 325 may be far focused. Notethat the first image plane covers upper and central portions of afield-of-view of a HMD (e.g., the HMD 200 in FIG. 2A), which naturallycorresponds to far focused vision. In some embodiments, the firstportion 330 of the bifocal optical element 325 may also provide opticalcorrection to the image light 340 to generate the optically correctedimage light 345 in the first image plane.

The electronic display 310 further emits image light 347 from anotherportion of the electronic display 310 located below the optical axis342, wherein the image light 347 may correspond to a near virtual imagedisplayed on this lower portion of the electronic display 310. The imagelight 347 may be optically corrected by the second portion 335 of thebifocal optical element 325 and then directed to at least one surface ofthe eye 320 as optically corrected image light 350. Thus, the imagelight 350 provides the near virtual image to the eye 320 in a secondimage plane, wherein the eye 320 looking through the second portion 335of the bifocal optical element 325 may be near focused. Note that thesecond image plane covers a lower portion of the field-of-view of theHMD, which naturally corresponds to near focused vision.

The first portion 330 of the bifocal optical element 325 may have afirst optical power, whereas the second portion 335 of the bifocaloptical element 325 may have a second optical power different than thefirst optical power. A radius of curvature of the first portion 330 ofthe bifocal optical element 325 may be different than that of the secondportion 335 of the bifocal optical element 325, resulting into differentoptical powers for the first portion 330 and the second portion 335 ofthe bifocal optical element 325. The bifocal optical element 325 mayprovide optical correction to the image light 340 determined by thefirst optical power of the first portion 330 of the bifocal opticalelement 325. The bifocal optical element 325 may further provide opticalcorrection to the image light 347 determined by the second optical powerof the second portion 335 of the bifocal optical element 325.

In some embodiments, the second optical power of the second portion 335of the bifocal optical element 325 is smaller than the first opticalpower of the first portion 330 of the bifocal optical element 325.Therefore, the second portion 335 of the bifocal optical element 325provides optical correction to the image light 347 by reducing anoptical power when the image light 347 propagates through the secondportion 335 of the bifocal optical element 325. The second portion 335of the bifocal optical element 325 may be thus implemented as a powerreducer. In some embodiments, the second portion 335 of the bifocaloptical element 325 has a negative optical power, whereas the firstportion 330 of the bifocal optical element 325 has zero optical power.Content presented through the second portion 335 of the bifocal opticalelement 325 (i.e., power reducer) reduces an accommodative range of theimage light 350 optically corrected by the second portion 335 of thebifocal optical element 325. Thus, the optically corrected image light350 has a smaller accommodative range than the image light 345propagated by the first portion 330 of the bifocal optical element 325.

In accordance with embodiments of the present disclosure, as discussed,the bifocal optical element 325 allows users of different accommodativeranges to view content presented on the electronic display 310 in twodifferent image planes, i.e., the first image plane for content viewedthrough the first portion 330 of the bifocal optical element 325 and thesecond image plane for other content viewed through the second portion335 of the bifocal optical element 325. The second portion 335 of thebifocal optical element 325 achieves this by setting an accommodativerange between the first and second image plane such that a broader rangeof users (e.g., children and adults) are able to focus on either imageplane. In this way, the bifocal optical element 325 generates twoseparate image planes that are at different image distances. Usershaving different ranges of accommodation (e.g., users of different ages)are able to focus on both of the image planes, thereby expanding a sizeof a user base for a HMD (e.g., the HMD 200 of FIG. 2A). Moreover, thebifocal optical element 325 is a passive device. Thus, the HMD with theoptical assembly 305 having the bifocal optical element 325 does notrequire any moving parts to achieve different ranges of accommodationfor various users.

In some embodiments, as shown in FIG. 3A, the second portion 335 of thebifocal optical element 325 (i.e., power reducer) is positioned belowthe optical axis 342. Thus, the user wearing the HMD 200 gazes below theoptical axis 342 and through the second portion 335 of the bifocaloptical element 325 to reduce an optical power of content presented on anear focal area of the electronic display 310. On the other hand, theuser wearing the HMD 200 gazes above the optical axis 342 and throughthe first portion 330 of the bifocal optical element 325 to preventreducing an optical power of other content presented on a far focal areaof the electronic display 310. In this way, users of differentaccommodative ranges are allowed to view content in at least two imageplanes as the bifocal optical element 325 directs the far focused imagelight 345 in the first image plane and the near focused image light 350in the second image plane. The first image plane covering a portion ofthe HMD's field-of-view below the optical axis 342 is thereforeassociated with the optically corrected image light 345 viewed throughthe first portion 330 of the bifocal optical element 325. Similarly, thesecond image plane covering other portions of the HMD's field-of-view isassociated with the image light 350 viewed through the second portion335 of the bifocal optical element 325.

A user of a HMD may gaze through the second portion 335 of the bifocaloptical element 325 when looking at content that appear near when beingrendered and displayed on the electronic display 310. In this case, theuser's eye 320 is near focused. As the second portion 335 of the bifocaloptical element 325 may be implemented as a power reducer having anegative optical power, a focal length d_(f) for the eye 320 when gazinginto the content that appear near on the electronic display 310 isdecreased and may be equal to or approximately equal to a vergence depthd_(v) associated with the near content. In a following time period, theuser of the HMD may gaze outside the second portion 335 of the bifocaloptical element 325 (i.e., outside the power reducer) and through thefirst portion 330 of the bifocal optical element 325 (which may not haveany optical power) when looking into other content that appear far whenbeing rendered and displayed on the electronic display 310. Because ofthat, a focal length d_(f) of the eye 320 decreases and the eye 320 isfar focused. Eventually, the focal length d_(f) of the eye 320 gazing atthe other content that appear far when displayed on the electronicdisplay 310 may be equal to or approximately equal to a vergence depthd_(v) that also increase as the other content appear far on theelectronic display 310. Thus, due to the bifocal optical element 325providing at least two image planes the accommodation process is coupledwith the vergence process, which mitigates vergence-accommodationconflict in relation to the user's eye 320.

As shown in FIG. 3A, the second portion 335 of the bifocal opticalelement 325 may be surrounded by the first portion 335 of the bifocaloptical element 325. In one embodiment, the bifocal optical element 325is implemented as a single lens, wherein the second portion 335 of thesingle lens has a different optical power (e.g., lower optical power)than the first portion 330 of the single lens. The second portion 335 ofthe bifocal optical element 325 may be composed of a different materialthan the first portion 330 of the bifocal optical element 325. In someembodiments, the bifocal optical element 325 is replaceable and selectedfrom a set of bifocal optical elements. Each bifocal optical elementfrom the set may have a different and unique combination of the firstoptical power and the second optical power. The bifocal optical element325 can be spherical, aspherical, consisting of a polynomial basis, orof a free-form.

In some embodiments, the bifocal optical element 325 comprises multiplelenses, each lens having a different optical power. For example, thefirst portion 330 of the bifocal optical element 325 comprises a firstlens, and the second portion 335 of the bifocal optical element 325comprises a second lens coupled to the first lens. In one embodiment(not shown in FIG. 3A), there is an air gap between the first lens andthe second lens of the bifocal optical element 325. In other embodiment(not shown in FIG. 3A), the second portion 335 of the bifocal opticalelement 325 implemented as a lens separate from the first portion 330can be positioned on at least a portion of a surface of the electronicdisplay 310, e.g., on a near focal portion of the electronic display310.

In some embodiments, the bifocal optical element 325 is implemented as adynamic lens, e.g., a liquid crystal lens, having two operationalstates—an active state and an inactive state. For example, the bifocaloptical element 325 implemented as a dynamic lens can be in the activestate only over the second portion 335, i.e., only over a lower portionof the field-of-view. When the bifocal optical element 325 (e.g., liquidcrystal lens) is in the active state, the bifocal optical element 325may provide, via the second portion 335, a negative optical power to thelower portion of the field-of-view and to the image light 347. In thisway, when being in the active state, the bifocal optical element 325implemented as a dynamic lens directs the near focused image light 350in the second image plane. On the other hand, when the bifocal opticalelement 325 (e.g., liquid crystal lens) is in the inactive state, thebifocal optical element 325 may provide no optical power, e.g., to theimage light 340 associated with the upper portion of the field-of-view.In this way, when being in the inactive state, the bifocal opticalelement 325 implemented as a dynamic lens directs the far focused imagelight 345 in the first image plane. In some embodiments, a controllercoupled to the bifocal optical element 325 (not shown in FIG. 3A) may beconfigured to control an operational state of the bifocal opticalelement 325 implemented as a dynamic lens, e.g., based on informationabout a position and orientation of the user's eye 320 in the eye-box315 obtained from an eye tracker (not shown in FIG. 3A). For example,when the user is looking at a near virtual image in the lower portion ofthe field-of-view, the controller may use information about a positionof the eye 320 from the eye tracker to activate the bifocal opticalelement 325 implemented as a dynamic lens. On the other hand, when theuser is looking at a far virtual image in the upper portion of thefield-of-view, the controller may use information about other positionof the eye 320 from the eye tracker to deactivate the bifocal opticalelement 325 implemented as a dynamic lens.

FIG. 3B shows examples of bifocal optical elements 325 a-c, inaccordance with one or more embodiments. The bifocal optical elements325 a-c are embodiments of the bifocal optical element 325 in FIG. 3A.As shown in FIG. 3B, an aperture of the bifocal optical element 325 ofFIG. 3A can be divided in several ways. In a first example 355 in FIG.3B, the second portion 335 a of the bifocal optical element 325 aoccupies a portion of a lower half of the bifocal optical element 325 a.The first portion 330 a occupies the remaining portion of the bifocaloptical element 325 a. A cross-section of the second portion 335 a inthe first example 355 represents a portion of a circle. In a secondexample 360 in FIG. 3B, the second portion 335 b of the bifocal opticalelement 325 b occupies the entire lower half of the bifocal opticalelement 325 b. In this case, the entire upper half of the bifocaloptical element 325 b is occupied by the first portion 330 b. In a thirdexample 365 in FIG. 3B, the second portion 335 c of the bifocal opticalelement 325 c occupies a portion of a lower half of the bifocal opticalelement 325 c, wherein a cross-section of the second portion 335 crepresents a full circle. The first portion 330 c occupies the remainingportion of the bifocal optical element 325 c.

FIG. 3C shows an example cross-section 370 of a bifocal optical element325 d and propagation of image light through the bifocal optical element325 d for a near-focused eye, in accordance with one or moreembodiments. The bifocal optical element 325 d including the firstportion 330 d and the second portion 335 d is an embodiment of thebifocal optical element 325 in FIG. 3A. In some embodiments, asdiscussed, the second portion 335 d is implemented as a lens having anegative optical power. The first portion 330 d may occupy a remainingportion of the bifocal optical element 325 d not being occupied by thesecond portion 335 d. In some embodiments, as shown in FIG. 3C, aFresnel lens 372 is positioned in optical series with the bifocaloptical element 325 d.

The image light 347 emitted from a near focal area of the electronicdisplay 310 (not shown in FIG. 3C) is optically corrected by the secondportion 335 d of the bifocal optical element 325 d so as a lowerfield-of-view of the HMD is focused to a near virtual image. As the eye320 is near focused, the image light 340 emitted from a far focal areaof the electronic display 310 propagating through the first portion 330d of the bifocal optical element 325 d is not in focus at the eye 320.Thus, an upper field-of-view of the HMD is not focused to a far virtualimage.

FIG. 3D shows an example cross-section 375 of the bifocal opticalelement 325 e and propagation of image light through the bifocal opticalelement 325 e for a far-focused eye, in accordance with one or moreembodiments. The bifocal optical element 325 e including the firstportion 330 e and the second portion 335 e is an embodiment of thebifocal optical element 325 in FIG. 3A. As discussed, the second portion335 e may implemented as a lens having a negative optical power. Thefirst portion 330 e may occupy a remaining portion of the bifocaloptical element 325 e not being occupied by the second portion 335 e. Insome embodiments, as shown in FIG. 3D, a Fresnel lens 377 is positionedin optical series with the bifocal optical element 325 e.

The image light 340 emitted from a far focal area of the electronicdisplay 310 (not shown in FIG. 3D) is propagated by the first portion330 e of the bifocal optical element 325 e so as an upper field-of-viewof the HMD is focused to a far virtual image. As the eye 320 is now farfocused, the image light 347 emitted from a near focal area of theelectronic display 310 (not shown in FIG. 3D) propagating through thesecond portion 335 e of the bifocal optical element 325 e is not infocus at the eye 320. Thus, a lower field-of-view of the HMD is notfocused to a near virtual image.

FIG. 4 is an example design of a multi-focal optical element 400implemented as a progressive lens, in accordance with one or moreembodiments. In some embodiments, the multi-focal optical element 400can replace the bifocal optical element 325 as part of the opticalassembly 305 of FIG. 3A. FIG. 4 shows a plurality of zones of themulti-focal optical element 400, each zone having a different opticalpowers. Note that a range of diopters and a layout of the zones of themulti-focal optical element 400 can be chosen in accordance withpreferred optical corrections provided by the multi-focal opticalelement 400. In some embodiments (not shown in FIG. 4), the multi-focaloptical element 400 is designed with a smoothly varying optical poweracross a surface of the multi-focal optical element 400.

FIG. 5 is an example cross section 500 of an optical assembly 505coupled to an electronic display 510, in accordance with one or moreembodiments. The optical assembly 505 may be an embodiment of theoptical assembly 250 of FIG. 2B. The optical assembly 505 may include abifocal insert 515 in optical series with at least one optical element(lens) 520. The bifocal insert 515 is an optical plate that includes alens 525, which may be implemented as a power reducer having a negativeoptical power in a lower region of a field-of-view of a HMD (e.g., theHMD 200 in FIG. 2A) position below an optical axis 527. The bifocalinsert 515 may have no optical power in other portions of thefield-of-view of the HMD.

The electronic display 510 coupled to the optical assembly 505 may beconfigured to emit image light 535 associated with a far focal portionof the electronic display 510 and image light 535 associated with a nearfocal portion of the electronic display 510. The electronic display 510may be an embodiment of the electronic display 245 of FIG. 2B. The lens(power reducer) 525 may provide optical correction to the image light535 related to the near focal portion and directs optically correctedimage light 540 in a near-focused image plane to a user's eye 545. Otherportions of the bifocal insert 515 may be configured to direct the imagelight 530 related to the far focal portion as image light 550 in afar-focused image plane to the eye 550.

In some embodiments, the bifocal insert 515 can be combined with the atleast one optical element 520 to achieve an appropriate opticalcorrection and visual experience for a user. An optical power of the atleast one optical element 520 may be selected based on, e.g., an opticalpower of the power reducer 525 and/or an optical power of the remainingportion of the bifocal insert 515. In some embodiments, the bifocaloptical element 325 of FIG. 3 can be integrated into the opticalassembly 505 as the bifocal insert 515. In other embodiments, themulti-focal optical element 400 of FIG. 4 implemented as a progressivelens can be integrated into the optical assembly 505 instead of thebifocal insert 515.

FIG. 6A is an example cross section 600 of an optical assembly 605 inoptical series with a tilted display 610 for achieving lower fieldmyopia, in accordance with one or more embodiments. The optical assembly605 in optical series with the display 610 may be part of the HMD 200 inFIG. 2A. The optical assembly 605 may include an optical element, e.g.,a convex lens 615 as shown in FIG. 6A. In some embodiments, the opticalassembly 605 may include other or additional optical elements (lenses).The optical assembly 605 may be an embodiment of the optical assembly250 of the front rigid body 205 of FIG. 2B.

The display 610 can be implemented as an electronic display that istilted relative to an optical axis 620. The display 610 may be anembodiment of the electronic display 245 of the front rigid body 205 ofFIG. 2B. By tilting the display 610 relative to the optical axis 620,the display 610 in optical series with the lens 615 provides two imageplanes with different focuses to a user's eye 625. The display 610 canbe divided into a near focal portion and a far focal portion to mitigatevergance-accommodation conflict in both users with a large range ofaccommodation as well as users with a more limited range ofaccommodation.

The display 610 emits image light 622 from a near focal portion of thedisplay 610 that presents content in a lower field-of-view of a HMD(e.g., the HMD 200 in FIG. 2A) positioned below the optical axis 620.The near focal portion of the display 610 is located below the opticalaxis 620 and features locally decreased optical power due to the tiltingof the display 610. The image light 622 emitted from the display 610 maybe optically corrected by the lens 615 before reaching at least onesurface of the eye 625. In this way, the lower field-of-view is focusedto a near virtual image 627 related to content being presented in thenear focal portion of the display 610. Note that humans naturally havelower field myopia, i.e., they naturally look in a lower field-of-viewfor near objects (e.g., objects within an arm length). The tilteddisplay 610 in optical series with the lens 615 provides lower fieldmyopia in artificial reality systems.

The display 610 further emits image light 630 from a far focal portionof the display 610 that presents content in an upper field-of-view ofthe HMD (e.g., above the optical axis 620). As discussed above, an upperfield-of-view is naturally used by humans for looking into far objects.The far focal portion of the display 610 is located above the opticalaxis 620 and features non-modified optical power due to the tilting ofthe display 610. The image light 630 emitted from the display 610 may beoptically corrected by the lens 615 before reaching at least one surfaceof the eye 625. In this way, the upper field-of-view is focused to a farvirtual image 632 related to content presented in the far focal portionof the display 610.

By tilting the display 610 along the optical axis 620, the display 610is positioned with respect to the optical axis 620 such that the imagelight 622 emitted by the near focal portion of the display 610 and theimage light 630 emitted by the far focal portion of the display 610appear to originate at different distances from the optical assembly605. In this way, the optical assembly 605 generates at least a firstimage plane associated with the far focal portion of the display 610 anda second image plane associated with the near focal portion of thedisplay 610. By providing multiple image planes associated with near andfar focal distances to the user's eye 625, the accommodation process iscoupled with the vergence process, which mitigatesvergence-accommodation conflict in relation to the user's eye 625.

FIG. 6B is an example cross section 635 of the optical assembly 605 withthe lens 615 in optical series with a curved (bent) display 640 forachieving lower field myopia, in accordance with one or moreembodiments. The optical assembly 605 in optical series with the display640 may be part of the HMD 200 in FIG. 2A. The display 640 can beimplemented as an electronic display that is curved or bent in at leastone dimension relative to the optical axis 620. By curving or bendingthe display 640 relative to the optical axis 620, the display 640 inoptical series with the lens 615 provides two image planes withdifferent focuses to mitigate vergance-accommodation conflict for avariety of users. The display 640 may be an embodiment of the electronicdisplay 245 of the front rigid body 205 of FIG. 2B.

The display 640 emits image light 642 from a near focal portion of thedisplay 640 that presents content in a lower field-of-view (e.g., belowthe optical axis 620). The near focal portion of the display 640 islocated below the optical axis 620 and features locally decreasedoptical power due to the curving or bending of the display 610. Theimage light 642 emitted from the display 640 may be optically correctedby the lens 615 before reaching at least one surface of the eye 625. Inthis way, the lower field-of-view is focused to a near virtual image 645related to content being presented in the near focal portion of thedisplay 640. In this way, the curved or bent display 640 in opticalseries with the lens 615 provides lower field myopia in artificialreality systems.

The display 640 further emits image light 647 from a far focal portionof the display 640 in an upper field-of-view of the HMD (e.g., above theoptical axis 620), which may be used for content that appears furtheraway. The image light 647 emitted from the display 640 may be opticallycorrected by the lens 615 before reaching at least one surface of theeye 625. In this way, the upper field-of-view is focused to a farvirtual image 650 related to content being presented in the far focalportion of the display 640.

By curving or bending the display 614 along the optical axis 620, thedisplay 640 is positioned with respect to the optical axis 620 such thatthe image light 642 emitted by the near focal portion of the display 640and the image light 647 emitted by the far focal portion of the display640 appear to originate at different distances from the optical assembly605. In this way, the optical assembly 605 generates at least a firstimage plane associated with the far focal portion of the display 640 anda second image plane associated with the near focal portion of thedisplay 640. By providing multiple image planes associated with near andfar focal distances to the user's eye 625, the accommodation process iscoupled with the vergence process, which mitigatesvergence-accommodation conflict in relation to the user's eye 625.

FIG. 7A is an example cross section 700 of an optical assembly 705 inoptical series with a display 710 having a wedge element 715 forachieving lower field myopia, in accordance with one or moreembodiments. The optical assembly 705 in optical series with the display710 may be part of the HMD 200 in FIG. 2A. The optical assembly 705 mayinclude an optical element, e.g., a convex lens 720, as shown in FIG.7A. In some embodiments, the optical assembly 705 may include other oradditional optical elements (lenses). The optical assembly 705 may be anembodiment of the optical assembly 250 of the front rigid body 205 ofFIG. 2B.

The display 710 may be implemented as an electronic display having thewedge element 715 coupled to a near focal portion of the display 710corresponding to a lower-field-of-view of a HMD (e.g., the HMD 200 inFIG. 2A) positioned below an optical axis 730. The wedge element 715 maybe implemented as a wedged piece of glass or plastic coupled to a lowerportion of the display 710. The wedge element 715 may change an opticaldepth in the lower portion of the display 710 and may provide alocalized shift in virtual image distance. Thus, by coupling the wedgeelement 715 to the lower portion of the display 710, the display 710 inoptical series with the lens 720 provides two image planes withdifferent focuses to mitigate vergance-accommodation conflict for avariety of users. The display 710 may be an embodiment of the electronicdisplay 245 of the front rigid body 205 of FIG. 2B.

The display 710 emits image light 735 from the near focal portion of thedisplay 710 coupled to the wedge element 715 that presents content in alower field-of-view of the HMD. The image light 735 may be opticallycorrected by the lens 720 before reaching at least one surface of theeye 725. In this way, the lower field-of-view is focused to a nearvirtual image 740 related to content being presented in the near focalportion of the display 710. In this way, the display 710 in opticalseries with the lens 720 provides lower field myopia in artificialreality systems.

The display 710 further emits image light 745 from a far focal portionof the display 710 in an upper field-of-view (e.g., above the opticalaxis 730), which may be used for virtual content that appears furtheraway. The image light 745 emitted from the display 710 may be opticallycorrected by the lens 720 before reaching at least one surface of theeye 725. In this way, the upper field-of-view is focused to a farvirtual image 750 related to content being presented in the far focalportion of the display 710.

By coupling the wedge element 715 to the display 710, the display 710 ispositioned with respect to the optical axis 730 such that the imagelight 735 emitted by the near focal portion of the display 710 and theimage light 745 emitted by the far focal portion of the display 710appear to originate at different distances from the optical assembly705. In this way, the optical assembly 705 generates at least a firstimage plane associated with the far focal portion of the display 710 anda second image plane associated with the near focal portion of thedisplay 710. The wedge element 715 coupled as a secondary element to thedisplay 710 is configured to adjust the image light 735 emitted from thenear focal portion of the display 710 located below the optical axis 730such that the image light 735 appears at the second image plane. Byproviding multiple image planes associated with near and far focaldistances to the user's eye 725, the accommodation process is coupledwith the vergence process, which mitigates vergence-accommodationconflict in relation to the user's eye 725.

FIG. 7B is an example cross section 755 of the optical assembly 705 inoptical series with a display 760 having a wedge element 765 forachieving lower field myopia, in accordance with one or moreembodiments. The optical assembly 705 in optical series with the display760 may be part of the HMD 200 in FIG. 2A.

The display 760 can be implemented as an electronic display having thewedge 765 coupled to a near focal portion of the display 760, e.g., to aportion of the display below the optical axis 730. The display 760 maybe an embodiment of the electronic display 245 of the front rigid body205 of FIG. 2B. The wedge element 765 may be implemented as a wedgedpiece of glass or plastic coupled to the lower portion of the display760. As shown in FIG. 7B, the wedge element 765 can be implemented tothin out near a very bottom of the near focal portion of the display760. In this way, content presented in a ground region of the display760 may not appear optically close. Similarly as for the wedge element715 of FIG. 7A, the wedge element 765 may change an optical depth in thenear focal portion of the display 760 and may provide a localized shiftin virtual image distance. Thus, by coupling the wedge element 765 tothe near focal portion of the display 760, the display 760 in opticalseries with the lens 720 provides two image planes with differentfocuses to mitigate vergance-accommodation conflict for a variety ofusers.

The display 760 emits image light 770 from the near focal portion of thedisplay 760 coupled to the wedge element 765 that presents content in alower field-of-view of the HMD (e.g., below the optical axis 730). Theimage light 770 may be optically corrected by the lens 720 beforereaching at least one surface of the eye 725. In this way, the lowerfield-of-view is focused to a near virtual image 775 related to contentbeing presented in the near focal portion of the display 760. Thus, thedisplay 760 in optical series with the lens 720 provides lower fieldmyopia in artificial reality systems.

The display 760 further emits image light 780 from a far focal portionof the display 760 in an upper field-of-view of the HMD (e.g., above theoptical axis 730), which may be used for content that appears furtheraway. The image light 780 emitted from the display 760 may be opticallycorrected by the lens 720 before reaching at least one surface of theeye 725. In this way, the upper field-of-view is focused to a farvirtual image 785 related to content being presented in the far focalportion of the display 760.

By implementing the display 760 with the wedge element 765, the display760 is positioned with respect to the optical axis 730 such that theimage light 770 emitted by the near focal portion of the display 760 andthe image light 780 emitted by the far focal portion of the display 760appear to originate at different distances from the optical assembly705. In this way, the optical assembly 705 generates at least a firstimage plane associated with the far focal portion of the display 760 anda second image plane associated with the near focal portion of thedisplay 760. The wedge element 765 coupled as a secondary element to thedisplay 760 is configured to adjust the image light 770 emitted from thenear focal portion of the display 760 located below the optical axis 730such that the image light 770 appears at the second image plane. Notethat a location of the second image plane is based in part on athickness of the wedge element 756. By providing multiple image planesassociated with near and far focal distances to the user's eye 725, theaccommodation process is coupled with the vergence process, whichmitigates vergence-accommodation conflict in relation to the user's eye725.

FIG. 8 is an example cross section 800 of an optical assembly 805 inoptical series with a display 810 coupled to an optical element 815 forachieving lower field myopia, in accordance with one or moreembodiments. The optical assembly 805 in optical series with the display810 may be part of the HMD 200 in FIG. 2A. The optical assembly mayprovide optical correction to image light emitted from differentportions of the display 810, and may direct the optically correctedimage light to an eye-box 820 where a user's eye 825 is located. Theoptical assembly 805 may include a lens 830 for optical correction ofthe image light. In some embodiments, the optical assembly 805 mayinclude other or additional optical elements (lenses). The opticalassembly 805 may be an embodiment of the optical assembly 250 of thefront rigid body 205 of FIG. 2B.

The display 810 can be implemented as an electronic display. The display810 may be an embodiment of the electronic display 245 of the frontrigid body 205 of FIG. 2B. A near focal portion of the display 810corresponding to a lower field-of-view of a HMD (e.g., the HMD 200 inFIG. 2A) may be coupled to the optical element 815. In some embodiments,the optical element 815 is implemented as a rectangular prism. Therectangular prism includes an output surface that is substantially flat.An increase of thickness in the near focal portion of the display 810coupled to the optical element 815 may change an optical depth in thisportion of the display 810, and may provide a localized shift in virtualimage distance. In this way, the portion of the display 810 coupled tothe optical element 815 provides a better focus for a near virtualobject. By coupling the optical element 815 to the near focal portion ofthe display 810, the display 810 in optical series with the lens 830provides two image planes with different focuses to mitigatevergance-accommodation conflict for a variety of users.

The display 810 emits image light 835 from the near focal portion of thedisplay 810 coupled to optical element 815, wherein the image light 835may be related to content being presented in a lower field-of-view ofthe HMD. The image light 835 may be optically corrected by the lens 820before reaching at least one surface of the eye 825. In this way, thelower field-of-view is focused to a near virtual image related tocontent being presented in the near focal portion of the display 810.Therefore, the display 810 in optical series with the lens 820 provideslower field myopia in artificial reality systems.

The display 810 further emits image light 840 from a far focal portionof the display 810 related to an upper field-of-view of the HMD, whichmay be used for presenting virtual content that appear further away. Theimage light 840 emitted from the display 810 may be optically correctedby the lens 820 before reaching at least one surface of the eye 825. Inthis way, the upper field-of-view is focused to a far virtual imagerelated to content being presented in the far focal portion of thedisplay 810.

By coupling the optical element 815 to the display 810, the display 810is positioned with respect to an optical axis such that the image light835 emitted by the near focal portion of the display 810 and the imagelight 840 emitted by the far focal portion of the display 810 appear tooriginate at different distances from the optical assembly 805. In thisway, the optical assembly 805 generates at least a first image planeassociated with the far focal portion of the display 810 and a secondimage plane associated with the near focal portion of the display 810.The optical element 815 coupled as a secondary element to the display810 is configured to adjust the image light 835 emitted from the nearfocal portion of the display 810 located below the optical axis suchthat the image light appears at the second image plane. By providingmultiple image planes associated with near and far focal distances tothe user's eye 825, the accommodation process is coupled with thevergence process, which mitigates vergence-accommodation conflict inrelation to the user's eye 825.

FIG. 9 is an example cross section 900 of an optical assembly 905 inoptical series with a display 910 having a wedged or curved fiber opticfaceplate 915 for achieving lower field myopia, in accordance with oneor more embodiments. The optical assembly 905 in optical series with thedisplay 910 may be part of the HMD 200 in FIG. 2A. The optical assembly905 may include an optical element, e.g., a convex lens 920, as shown inFIG. 9. In some embodiments, the optical assembly 905 may include otheror additional optical elements (lenses). The optical assembly 905 may bean embodiment of the optical assembly 250 of the front rigid body 205 ofFIG. 2B.

The display 910 can be implemented as an electronic display having thewedged or curved fiber optic faceplate 915 bonded to at least a portionof the display 910, e.g., a near focal portion of the display 910positioned below an optical axis 925. The fiber optic faceplate 915 maybe implemented as a bundle of fibers that are bonded together such thatlight enters one side of the bundle and exits at the other. The fiberoptic faceplate 915 may include an input surface and an output surface(not shown in FIG. 9), the input surface receives image light emittedfrom the near focal portion of the display 910 and the output surfaceemits the received image light as the image light 935. Note that athickness of a portion of the fiber optic faceplate 915 that receivesthe image light from the near focal portion of the display 910 isgreater than a thickness of a portion of the fiber optic faceplate 915that receives image light from a far focal portion of the displaypositioned above the optical axis 925. In some embodiments, the fiberoptic faceplate 915 may be implemented to be of any suitable shape.

Similar as curving or bending a display shown in FIGS. 6A-6B, the fiberoptic faceplate 915 may change an optical depth in a portion of thedisplay 910 bonded to the fiber optic faceplate 915 providing alocalized shift in virtual image distance. By coupling the fiber opticfaceplate 915 on at least a portion of the display 910, the display 910in optical series with the lens 920 provides two image planes withdifferent focuses to mitigate vergance-accommodation conflict for avariety of users. In some embodiments, a near focal portion of thedisplay 910 related to a lower area of a field-of-view of a HMD (e.g.,the HMD 200 in FIG. 2A) may be implemented having an increased thicknessto provide a localized shift in virtual image distance and a betterfocus for near objects. The display 910 may be an embodiment of theelectronic display 245 of the front rigid body 205 of FIG. 2B.

The display 910 emits image light 935 from a near focal portion of thedisplay 910 bonded to the fiber optic faceplate 915 and positioned belowthe optical axis 925 to present content in a lower field-of-view of theHMD (e.g., below the optical axis 925). In some embodiments, asdiscussed, the near focal portion of the display 910 may have anincreased thickness. The image light 935 may be optically corrected bythe lens 920 before reaching at least one surface of the eye 930. Inthis way, the lower field-of-view is focused to a near virtual image 940related to content being presented in the near focal portion of thedisplay 910. Thus, the display 910 in optical series with the lens 920provides lower field myopia in artificial reality systems.

The display 910 having the fiber optic faceplate 915 further emits imagelight 945 from a far focal portion of the display 910 in an upperfield-of-view of the HMD (e.g., above the optical axis 925), which maybe used for presenting virtual content that appear further away. Theimage light 945 emitted from the display 910 may be optically correctedby the lens 920 before reaching at least one surface of the eye 930. Inthis way, the upper field-of-view is focused to a far virtual image 750related to content being presented in the far focal portion of thedisplay 910.

By implementing the display 910 with the fiber optic faceplate 915, thedisplay 910 is positioned with respect to the optical axis 925 such thatthe image light 935 emitted by the near focal portion of the display 910and the image light 945 emitted by the far focal portion of the display910 appear to originate at different distances from the optical assembly905. In this way, the optical assembly 905 generates at least a firstimage plane associated with the far focal portion of the display 910 anda second image plane associated with the near focal portion of thedisplay 910. The fiber optic faceplate 915 coupled as a secondaryelement to the display 910 is configured to adjust the image light 935emitted from the near focal portion of the display 910 located below theoptical axis 925 such that the image light 935 appears at the secondimage plane. A location of the second image plane may be based in parton a surface profile of the output surface of the fiber optic faceplate915. A location of the first image plane may be also based in part onthe surface profile of the output surface of the fiber optic faceplate915. By providing multiple image planes associated with near and farfocal distances to the user's eye 930, the accommodation process iscoupled with the vergence process, which mitigatesvergence-accommodation conflict in relation to the user's eye 930.

System Environment

FIG. 10 is a block diagram of one embodiment of a HMD system 1000 inwhich a console 1010 operates. The HMD system 1000 may operate in anartificial reality system. The HMD system 1000 shown by FIG. 10comprises a HMD 1005 and an input/output (I/O) interface 1015 that iscoupled to the console 1010. While FIG. 10 shows an example HMD system1000 including one HMD 1005 and on I/O interface 1015, in otherembodiments any number of these components may be included in the HMDsystem 1000. For example, there may be multiple HMDs 1005 each having anassociated I/O interface 1015, with each HMD 1005 and I/O interface 1015communicating with the console 1010. In alternative configurations,different and/or additional components may be included in the HMD system1000. Additionally, functionality described in conjunction with one ormore of the components shown in FIG. 10 may be distributed among thecomponents in a different manner than described in conjunction with FIG.10 in some embodiments. For example, some or all of the functionality ofthe console 1010 is provided by the HMD 1005.

The HMD 1005 is a head-mounted display that presents content to a usercomprising virtual and/or augmented views of a physical, real-worldenvironment with computer-generated elements (e.g., two-dimensional (2D)or three-dimensional (3D) images, 2D or 3D video, sound, etc.). In someembodiments, the presented content includes audio that is presented viaan external device (e.g., speakers and/or headphones) that receivesaudio information from the HMD 1005, the console 1010, or both, andpresents audio data based on the audio information. The HMD 1005 maycomprise one or more rigid bodies, which may be rigidly or non-rigidlycoupled together. A rigid coupling between rigid bodies causes thecoupled rigid bodies to act as a single rigid entity. In contrast, anon-rigid coupling between rigid bodies allows the rigid bodies to moverelative to each other. An embodiment of the HMD 1005 may be the HMD 200described above in conjunction with FIG. 2.

The HMD 1005 includes a DCA 1020, an electronic display 1025, an opticalassembly 1030, one or more position sensors 1035, an IMU 1040, anoptional eye tracking system 1045, and an optional varifocal module1050. Some embodiments of the HMD 1005 have different components thanthose described in conjunction with FIG. 10. Additionally, thefunctionality provided by various components described in conjunctionwith FIG. 10 may be differently distributed among the components of theHMD 1005 in other embodiments.

The DCA 1020 captures data describing depth information of a local areasurrounding some or all of the HMD 1005. The DCA 1020 can compute thedepth information using the data (e.g., based on a captured portion of astructured light pattern), or the DCA 1020 can send this information toanother device such as the console 1010 that can determine the depthinformation using the data from the DCA 1020. The DCA 1020 may be anembodiment of the DCA 340 in FIG. 3.

The electronic display 1025 displays two-dimensional orthree-dimensional images to the user in accordance with data receivedfrom the console 1010. In various embodiments, the electronic display1025 comprises a single electronic display or multiple electronicdisplays (e.g., a display for each eye of a user). Examples of theelectronic display 1025 include: a liquid crystal display (LCD), anorganic light emitting diode (OLED) display, an inorganic light emittingdiode (ILED) display, an active-matrix organic light-emitting diode(AMOLED) display, a transparent organic light emitting diode (TOLED)display, some other display, or some combination thereof.

In some embodiments, the electronic display 1025 is configured topresent content in two image planes. The electronic display electronicdisplay 1025 can be implemented to have a near and far focal portion tomitigate vergence-accommodation conflict in both users with a largerange of accommodation as well as users with a more limited range ofaccommodation. The near focal portion is generally positioned in a lowerportion of a field-of-view of the HMD 1005. In one embodiment, theelectronic display 1025 provides two image planes by tilting (orbending) the electronic display 1025 with respect to the opticalassembly 1030. In other embodiment, the electronic display 1025 providestwo image planes by bending the electronic display 1025 with respect tothe optical assembly 1030. In yet other embodiments, a fiber taper or awedge element (both of which may be shaped) that is coupled to theelectronic display 1025 may be used to provide a localized shift invirtual image distance. In some embodiments, the electronic display 1025may represent the electronic display 245 in FIG. 2B, the electronicdisplay 310 of FIG. 3A, the display 610 of FIG. 6A, the display 640 ofFIG. 6B, the display 710 of FIG. 7A, the display 760 of FIG. 7B, thedisplay 810 of FIG. 8, and/or the display 910 of FIG. 9.

The optical assembly 1030 magnifies image light received from theelectronic display 1025, corrects optical errors associated with theimage light, and presents the corrected image light to a user of the HMD1005. The optical assembly 1030 includes a plurality of opticalelements. Example optical elements included in the optical assembly 1030include: an aperture, a Fresnel lens, a convex lens, a concave lens, afilter, a reflecting surface, or any other suitable optical element thataffects image light. Moreover, the optical assembly 1030 may includecombinations of different optical elements. In some embodiments, one ormore of the optical elements in the optical assembly 1030 may have oneor more coatings, such as partially reflective or anti-reflectivecoatings.

Magnification and focusing of the image light by the optical assembly1030 allows the electronic display 1025 to be physically smaller, weighless and consume less power than larger displays. Additionally,magnification may increase the field-of-view of the content presented bythe electronic display 1025. For example, the field-of-view of thedisplayed content is such that the displayed content is presented usingalmost all (e.g., approximately 110 degrees diagonal), and in some casesall, of the field-of-view. Additionally in some embodiments, the amountof magnification may be adjusted by adding or removing optical elements.

In some embodiments, the optical assembly 1030 may be designed tocorrect one or more types of optical error. Examples of optical errorinclude barrel or pincushion distortions, longitudinal chromaticaberrations, or transverse chromatic aberrations. Other types of opticalerrors may further include spherical aberrations, chromatic aberrationsor errors due to the lens field curvature, astigmatisms, or any othertype of optical error. In some embodiments, content provided to theelectronic display 1025 for display is pre-distorted, and the opticalassembly 1030 corrects the distortion when it receives image light fromthe electronic display 1025 generated based on the content.

In accordance with embodiments of the present disclosure, the opticalassembly 1030 includes a bifocal optical element that has a specificoptical power except for a portion of the bifocal optical element thatis formed to have less optical power (a power reducer). Contentpresented through the power reducer allow users of differentaccommodative ranges to view content in at least a first image plane anda second image plane, i.e., an image plane for content not viewedthrough the power reducer and an image plane for other content viewedthrough the power reducer. The power reducer sets an accommodative rangebetween the first and second image plane such that a broader range ofusers are able to focus on either image plane. The bifocal opticalelement of the optical assembly 1030 generates two separate image planesthat are at different image distances. Users having different ranges ofaccommodation are able to focus on both of the image planes, therebyexpanding a size of a user base for the HMD system 1000. Additionally,the bifocal optical element of the optical assembly 1030 may alsomitigate vergence-accommodation conflict. In some embodiments, theoptical assembly 1030 having the bifocal optical element may representthe optical assembly 250 in FIG. 2B and/or the optical assembly 305 ofFIG. 3A.

The IMU 1040 is an electronic device that generates data indicating aposition of the HMD 1005 based on measurement signals received from oneor more of the position sensors 1035 and from depth information receivedfrom the DCA 1020. A position sensor 1035 generates one or moremeasurement signals in response to motion of the HMD 1005. Examples ofposition sensors 1035 include: one or more accelerometers, one or moregyroscopes, one or more magnetometers, another suitable type of sensorthat detects motion, a type of sensor used for error correction of theIMU 1040, or some combination thereof. The position sensors 1035 may belocated external to the IMU 1040, internal to the IMU 1040, or somecombination thereof.

Based on the one or more measurement signals from one or more positionsensors 1035, the IMU 1040 generates data indicating an estimatedcurrent position of the HMD 1005 relative to an initial position of theHMD 1005. For example, the position sensors 1035 include multipleaccelerometers to measure translational motion (forward/back, up/down,left/right) and multiple gyroscopes to measure rotational motion (e.g.,pitch, yaw, roll). In some embodiments, the position sensors 1035 mayrepresent the position sensors 235 in FIG. 2. In some embodiments, theIMU 1040 rapidly samples the measurement signals and calculates theestimated current position of the HMD 1005 from the sampled data. Forexample, the IMU 1040 integrates the measurement signals received fromthe accelerometers over time to estimate a velocity vector andintegrates the velocity vector over time to determine an estimatedcurrent position of a reference point on the HMD 1005. Alternatively,the IMU 1040 provides the sampled measurement signals to the console1010, which interprets the data to reduce error. The reference point isa point that may be used to describe the position of the HMD 1005. Thereference point may generally be defined as a point in space or aposition related to the HMD's 1005 orientation and position.

The IMU 1040 receives one or more parameters from the console 1010. Theone or more parameters are used to maintain tracking of the HMD 1005.Based on a received parameter, the IMU 1040 may adjust one or more IMUparameters (e.g., sample rate). In some embodiments, certain parameterscause the IMU 1040 to update an initial position of the reference pointso it corresponds to a next position of the reference point. Updatingthe initial position of the reference point as the next calibratedposition of the reference point helps reduce accumulated errorassociated with the current position estimated the IMU 1040. Theaccumulated error, also referred to as drift error, causes the estimatedposition of the reference point to “drift” away from the actual positionof the reference point over time. In some embodiments of the HMD 1005,the IMU 1040 may be a dedicated hardware component. In otherembodiments, the IMU 1040 may be a software component implemented in oneor more processors. In some embodiments, the IMU 1040 may represent theIMU 230 in FIG. 2.

In some embodiments, the eye tracking system 1045 is integrated into theHMD 1005. The eye tracking system 1045 determines eye trackinginformation associated with an eye of a user wearing the HMD 1005. Theeye tracking information determined by the eye tracking system 1045 maycomprise information about an orientation of the user's eye, i.e.,information about an angle of an eye-gaze. In some embodiments, the eyetracking system 1045 is integrated into the optical assembly 1030. Anembodiment of the eye-tracking system 1045 may comprise an illuminationsource and an imaging device (camera).

In some embodiments, the varifocal module 1050 is further integratedinto the HMD 1005. The varifocal module 1050 may be coupled to the eyetracking system 1045 to obtain eye tracking information determined bythe eye tracking system 1045. The varifocal module 1050 may beconfigured to adjust focus of one or more images displayed on theelectronic display 1025, based on the determined eye trackinginformation obtained from the eye tracking system 1045. In this way, thevarifocal module 1050 can mitigate vergence-accommodation conflict inrelation to image light. The varifocal module 1050 can be interfaced(e.g., either mechanically or electrically) with at least one of theelectronic display 1025 and at least one optical element of the opticalassembly 1030. Then, the varifocal module 1050 may be configured toadjust focus of the one or more images displayed on the electronicdisplay 1025 by adjusting position of at least one of the electronicdisplay 1025 and the at least one optical element of the opticalassembly 1030, based on the determined eye tracking information obtainedfrom the eye tracking system 1045. By adjusting the position, thevarifocal module 1050 varies focus of image light output from theelectronic display 1025 towards the user's eye. The varifocal module1050 may be also configured to adjust resolution of the images displayedon the electronic display 1025 by performing foveated rendering of thedisplayed images, based at least in part on the determined eye trackinginformation obtained from the eye tracking system 1045. In this case,the varifocal module 1050 provides appropriate image signals to theelectronic display 1025. The varifocal module 1050 provides imagesignals with a maximum pixel density for the electronic display 1025only in a foveal region of the user's eye-gaze, while providing imagesignals with lower pixel densities in other regions of the electronicdisplay 1025. In one embodiment, the varifocal module 1050 may utilizethe depth information obtained by the DCA 1020 to, e.g., generatecontent for presentation on the electronic display 1025.

The I/O interface 1015 is a device that allows a user to send actionrequests and receive responses from the console 1010. An action requestis a request to perform a particular action. For example, an actionrequest may be an instruction to start or end capture of image or videodata or an instruction to perform a particular action within anapplication. The I/O interface 1015 may include one or more inputdevices. Example input devices include: a keyboard, a mouse, a gamecontroller, or any other suitable device for receiving action requestsand communicating the action requests to the console 1010. An actionrequest received by the I/O interface 1015 is communicated to theconsole 1010, which performs an action corresponding to the actionrequest. In some embodiments, the I/O interface 1015 includes an IMU1040 that captures IMU data indicating an estimated position of the I/Ointerface 1015 relative to an initial position of the I/O interface1015. In some embodiments, the I/O interface 1015 may provide hapticfeedback to the user in accordance with instructions received from theconsole 1010. For example, haptic feedback is provided when an actionrequest is received, or the console 1010 communicates instructions tothe I/O interface 1015 causing the I/O interface 1015 to generate hapticfeedback when the console 1010 performs an action.

The console 1010 provides content to the HMD 1005 for processing inaccordance with information received from one or more of: the DCA 1020,the HMD 1005, and the I/O interface 1015. In the example shown in FIG.10, the console 1010 includes an application store 1055, a trackingmodule 1060, and an engine 1065. Some embodiments of the console 1010have different modules or components than those described in conjunctionwith FIG. 10. Similarly, the functions further described below may bedistributed among components of the console 1010 in a different mannerthan described in conjunction with FIG. 10.

The application store 1055 stores one or more applications for executionby the console 1010. An application is a group of instructions, thatwhen executed by a processor, generates content for presentation to theuser. Content generated by an application may be in response to inputsreceived from the user via movement of the HMD 1005 or the I/O interface1015. Examples of applications include: gaming applications,conferencing applications, video playback applications, or othersuitable applications.

The tracking module 1060 calibrates the HMD system 1000 using one ormore calibration parameters and may adjust one or more calibrationparameters to reduce error in determination of the position of the HMD1005 or of the I/O interface 1015. For example, the tracking module 1060communicates a calibration parameter to the DCA 1020 to adjust the focusof the DCA 1020 to more accurately determine positions of structuredlight elements captured by the DCA 1020. Calibration performed by thetracking module 1060 also accounts for information received from the IMU1040 in the HMD 1005 and/or an IMU 1040 included in the I/O interface1015. Additionally, if tracking of the HMD 1005 is lost (e.g., the DCA1020 loses line of sight of at least a threshold number of structuredlight elements), the tracking module 1060 may re-calibrate some or allof the HMD system 1000.

The tracking module 1060 tracks movements of the HMD 1005 or of the I/Ointerface 1015 using information from the DCA 1020, the one or moreposition sensors 1035, the IMU 1040 or some combination thereof. Forexample, the tracking module 1050 determines a position of a referencepoint of the HMD 1005 in a mapping of a local area based on informationfrom the HMD 1005. The tracking module 1060 may also determine positionsof the reference point of the HMD 1005 or a reference point of the I/Ointerface 1015 using data indicating a position of the HMD 1005 from theIMU 1040 or using data indicating a position of the I/O interface 1015from an IMU 1040 included in the I/O interface 1015, respectively.Additionally, in some embodiments, the tracking module 1060 may useportions of data indicating a position or the HMD 1005 from the IMU 1040as well as representations of the local area from the DCA 1020 topredict a future location of the HMD 1005. The tracking module 1060provides the estimated or predicted future position of the HMD 1005 orthe I/O interface 1015 to the engine 1055.

The engine 1065 generates a 3D mapping of the area surrounding some orall of the HMD 1005 (i.e., the “local area”) based on informationreceived from the HMD 1005. In some embodiments, the engine 1065determines depth information for the 3D mapping of the local area basedon information received from the DCA 1020 that is relevant fortechniques used in computing depth. The engine 1065 may calculate depthinformation using one or more techniques in computing depth fromstructured light. In various embodiments, the engine 1065 uses the depthinformation to, e.g., update a model of the local area, and generatecontent based in part on the updated model.

The engine 1065 also executes applications within the HMD system 1000and receives position information, acceleration information, velocityinformation, predicted future positions, or some combination thereof, ofthe HMD 1005 from the tracking module 1060. Based on the receivedinformation, the engine 1065 determines content to provide to the HMD1005 for presentation to the user. For example, if the receivedinformation indicates that the user has looked to the left, the engine1065 generates content for the HMD 1005 that mirrors the user's movementin a virtual environment or in an environment augmenting the local areawith additional content. Additionally, the engine 1065 performs anaction within an application executing on the console 1010 in responseto an action request received from the I/O interface 1015 and providesfeedback to the user that the action was performed. The providedfeedback may be visual or audible feedback via the HMD 1005 or hapticfeedback via the I/O interface 1015.

In some embodiments, based on the eye tracking information (e.g.,orientation of the user's eye) received from the eye tracking system1045, the engine 1065 determines resolution of the content provided tothe HMD 1005 for presentation to the user on the electronic display1025. The engine 1065 provides the content to the HMD 1005 having amaximum pixel resolution on the electronic display 1025 in a fovealregion of the user's gaze, whereas the engine 1065 provides a lowerpixel resolution in other regions of the electronic display 1025, thusachieving less power consumption at the HMD 1005 and saving computingcycles of the console 1010 without compromising a visual experience ofthe user. In some embodiments, the engine 1065 can further use the eyetracking information to adjust where objects are displayed on theelectronic display 1025 to prevent vergence-accommodation conflict.

Additional Configuration Information

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the disclosure be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. A head-mounted display (HMD) comprising: anelectronic display configured to emit image light; and an opticalassembly configured to direct the image light to an eye-box of the HMDcorresponding to a location of a user's eye, the optical assemblycomprising a multifocal optical element, wherein: a first portion of themultifocal optical element has a first and non-negative optical powerthat is associated with a first image plane located at least above anoptical axis of the HMD, and a second portion of the multifocal opticalelement acting as an optical power reducer having a second and negativeoptical power across the entire second portion, the second portionassociated with a second image plane located below the optical axis, thesecond portion including a lens embedded into the multifocal opticalelement.
 2. The HMD of claim 1, wherein: the first image plane isassociated with a portion of the image light viewed through the firstportion of the multifocal optical element; and the second image plane isassociated with another portion of the image light viewed through thesecond portion of the multifocal optical element.
 3. The HMD of claim 1,wherein the second portion of the multifocal optical element issurrounded by the first portion of the multifocal optical element. 4.The HMD of claim 1, wherein the first portion of the multifocal opticalelement is composed of a different material than the second portion ofthe multifocal optical element.
 5. The HMD of claim 1, wherein themultifocal optical element comprises multiple zones, each zone having adifferent optical power.
 6. The HMD of claim 1, wherein the opticalassembly comprises at least one optical element in optical series withthe multifocal optical element.
 7. A head-mounted display (HMD)comprising: an electronic display configured to emit image light; and anoptical assembly configured to provide optical correction to the imagelight and direct the optically corrected image light to an eye-box ofthe HMD corresponding to a location of a user's eye, the opticalassembly comprising a multifocal optical element, wherein: a firstportion of the multifocal optical element has a first and non-negativeoptical power and is located at least above an optical axis of the HMD,a second portion of the multifocal optical element acting as an opticalpower reducer having a second and negative optical power across theentire second portion and is located below the optical axis, the secondportion including a lens embedded into the multifocal optical element,and the optical correction is determined in part by the first opticalpower and the second optical power.
 8. The HMD of claim 7, wherein theoptical assembly is configured to direct the optically corrected imagelight to the user's eye in at least two image planes.
 9. The HMD ofclaim 8, wherein: a first image plane of the at least two image planesis associated with a portion of the optically corrected image lightviewed through the first portion of the multifocal optical element; anda second image plane of the at least two image planes is associated withanother portion of the optically corrected image light viewed throughthe second portion of the multifocal optical element.
 10. The HMD ofclaim 7, wherein the multifocal optical element comprises a dynamiclens.
 11. The HMD of claim 7, wherein the multifocal optical elementcomprises multiple zones, each zone having a different optical power.12. The HMD of claim 7, wherein the optical assembly comprises at leastone optical element in optical series with the multifocal opticalelement.
 13. The HMD of claim 7, wherein the first portion of themultifocal optical element has zero optical power.
 14. The HMD of claim7, wherein the multifocal optical element is replaceable and selectedfrom a set of bifocal optical elements, each bifocal optical elementfrom the set having a different combination of the first optical powerand the second optical power.
 15. The HMD of claim 7, wherein themultifocal optical element is spherical, aspherical, consisting of apolynomial basis, or of a free-form.
 16. The HMD of claim 7, wherein thefirst portion of the multifocal optical element is composed of adifferent material than the second portion of the multifocal opticalelement.